This invention relates to a method and apparatus for producing large particles of material, and more particularly to a free space reactor for producing large particles greater than a few microns in diameter (preferably in the range of 10 to 100 microns) from a gas or gases.
There is a growing need for producing high-purity material inexpensively, such as silicon widely used for semiconductor devices, integrated circuits and solar cells. There are known techniques available for producing high-purity gases as intermediates from low grade material. These gases may then be used to produce high-purity material. However, known techniques for producing materials from gas, such as the Siemens process, are expensive batch processes. What is required is an inexpensive continuous process for producing material from gas, or gases.
A good example of the problem to be solved by the present invention is producing high-purity silicon from high-purity silane obtained as an intermediate from metallurgical grade silicon. At present most of the high grade purity silicon is produced by epitaxial reactors in the form of films, or in a bell jar or tubular reactors in bulk form. These are batch operation reactors with high energy and labor consumption. Consequently, the cost of production is high. What is required is a continuous, low-cost process.
Previous attempts by others to solve the problem have utilized a continuous flow reactor in which silicon aerosol is obtained from silane by thermal decomposition: EQU SiH.sub.4 .fwdarw.Si+2H.sub.2 ( 1)
as described by James R. Lay and Sridhar K. Iya, "Silane Pyrolysis in a Free-Space Reactor," Proc. 15th IEEE Photovoltaic Specialists Conference, pp 565-68 (June 1981). Because silane introduced through a port at the top of the reactor is immediately subjected to high temperature (1105.degree.-1285.degree. K.) within the reactor, homogeneous reaction occurs almost instantaneously as the silane enters the reactor. The silicon produced by the reaction nucleates and forms very fine silicon particles. These fine particles grow to a maximum size less than 1 micrometer by condensation of silicon and coagulation of the fine particles thus produced. The particles are continually filtered out at the bottom of the reactor as a powder.
Since silane can be fed to the reactor continuously, it would be possible to produce such high-purity silicon powder on a continuous basis. However, the powder is so fine, like lamp black, that subsequent processing is difficult. One method to obtain larger particles by coagulation and agglomeration is described by S. K. Friedlander in a paper titled "The Behavior of Constant Rate Aerosol Reactors," Aerosol Science and Technology 1:3-13 (1982). Friedlander describes a constant rate tubular flow aerosol reactor in which the reactant concentrations and reactor temperature remain approximately constant, and the rate of formation of aerosol material is also approximately constant. Such a reactor is suggested by Friedlander for application as a catalytic flow reactor since fresh surface area for catalytic reactions can be continually produced. The technique is to produce a large number of very small particles by homogenous nucleation of the products of gas phase reactions. The rate of chemical reaction which produces aerosol material is then controlled such that that fresh surface area is continually created by nucleation of new particles or growth of the existing particles. Nucleation continues as long as the stable clusters (small particles) are not present in sufficient concentration to scavenge the monomer molecules or smaller clusters. Once this critical point is passed, particles grow by vapor deposition, coagulation and agglomeration.
This technique of Friedlander is highly desirable if one's objective is the production of a high surface area to be used to promote catalytic chemical reactions. It is, however, not well suited to the production of bulk material because the number of small particles formed by nucleation in such a reactor is very large and coagulation is much too slow to achieve adequate growth of the particles thus produced within residence times practical for gas flow reactors. In contrast to the approach of Friedlander, the present invention limits the number of particles produced by nucleation in the reactor by allowing only a small amount of the reactant gas or gases entering the reactor to react sufficiently rapidly to form new particles by nucleation, then mixes those few seed particles with the primary reactant flow, and carries the primary reaction out at a slow rate to prevent any further nucleation. By this means, the present invention can quickly grow particles in excess of 10 microns in size without relying on the slow process of coagulation and agglomeration as primary growth mechanisms.
Another approach to the problem of producing silicon particles from pyrolytic reaction of silane involves the use of a "fluidized bed" in which a stream of silane and hydrogen flows through a bed of silicon particles. The flow of these gases suspend and agitate the particles to form a fluid-like bed, hence the term "fluidized bed." Silicon seed particles fed into a chamber near the top settle into the fluidized bed where they are heated to about 1075' K. Silane flowing up through the bed decomposes by pyrolytic reaction and deposits monomer molecules on the existing particles to make some particles grow larger. The larger particles thus grown precipitate out through the bottom of the bed, or are extracted from the bed. Some of the silane decomposes homogeneously leading to the formation of fine silicon powder. Larger particles filter out some of the smaller particles. This fluidized-bed approach produces larger particles than the free-space reactor described by Lay and Iya (supra), but it too may produce a fine powder of silicon that is too difficult to handle for subsequent processing.